I didn't see the one about using the CDs, but I'm using the post I believe SloSaturn used as a guideline. The HHR has the squared back end, so I was going to try the the basic 4in strake at a 15 to 20 degree angle along the edge. Likely have some on the sides also. Depending on how much material is left over, maybe even do some tuft testing on the hood.
Found a can of plastic spray paint at home. The safety angle was the deciding factor, but this helped. An inch height is recommended for the VG, so I'll check the tent stakes and corner protectors dimensions. I'm guessing the protectors will win out on price, but the tent stake tips would look better on the hood.
I opted for making mine out for .020 sheet aluminum and mounted them with 3M indoor/outdoor double sided tape which covers the entire bottom surface. These suckers are not going anywhere. I think the key it to prepare both mating surfaces properly. Dont paint the bottom surfave of the DIY VG's and throughly clean the rooftop surface with alcohol.
Yeah I did the tuft testing before and after installation. I was amazed at the tuft test results before installation. At 15-20 MPH the tufts started dancing around in all directions. At 25-30 the tufts actually started to move UP the window, opposite direction of "expected airflow". I only drove about 1 mile reaching 50 MPH before I realized just how messed up the airflow was on the back of my Mustang.
So after I installed the VG's and did the same drive test, they layed down perfectly as the sir was flowing over the back of the car like it should have been. So the VG's worked perfectly.
Since my VG's are kind of a DIY rip off of the Airtabs, with similar dimensions, I used the Airtabs specs for spacing. Spaced 4" on center. But I screwed up my count on the manufacturing. My roof width at the install point was 48". So I would have needed 11 VG's spaced 4" to cover the roof properly. At first I did 9 of the spaced at 4" and had a 6" dead zone on the drivvers side. Sure enough, I had some drivers side tufts that acted erradiicly. So I busted out the sheet aluminum, stencil and shears and made 1 more. I then respaced the 10 (should have been 11) to be like 4" then 4.25" then 4.5" on center to span the width of the roof.
As far as data...
I have only collected a small amount of data with the VG's. Only 4, 1/2 tank runs. So, only time will tell the real deal. As a side note, these 4, 1/2 tank runs were with my stock 5.0 Mustang spoiler removed. I did not want to damage it while standing on the back of the car installing the VG's.
Before VG's 22-27 MPG
After VG's 24-29 MPG
The results are promising. And I have noticed that the back of the car feels more stable, even without the spoiler,. I've had this Mustang for 10+ years and have completely rebuilt it from the ground up to be a road racer. However, it is my daily driver so a ANY minor difference in the handling or performance is something I pick up on right away.
I am new to the forum, but will keep with it and post any relevant results as they become available.